Constraining core-mantle interaction

在地球表面下约3000公里处，温度上升了2000K，周围的环境从固体硅酸盐矿物变成了液态铁合金。这就是核-地幔边界。这是我们星球上最深刻的不连续，在非常小的距离(200米)内发生了巨大的物理和化学变化。长期以来，边界上的温度差异一直被认为是地核上方地幔加热的原因，因此它变得不那么致密，像羽流一样上升，形成了夏威夷等海洋岛屿。然而，这种长期持有的“羽流模式”越来越受到质疑，需要一些确凿的证据来证明形成这些海洋岛屿的物质的最终深层来源。传统的想象地球内部的工具，地震学，很难清楚地分辨出海洋岛屿下狭窄的羽流。如果在洋岛熔岩中能够找到成分高度独特的核心旁边的居住地的化学印记，那么地球化学可以提供关键的证据。最近的研究表明，确实存在这样的签名。最初的工作重点是提高元素Os的同位素比值，即186Os/188Os。在夏威夷的熔岩中发现了高的186Os/188Os比值，这可以通过其源区岩芯的贡献来解释。然而，对于这一观察结果，还有其他看似合理的解释。另一个不那么模棱两可的指纹来自另一个同位素比，这次是W元素，182W/184W。这一比率只在地球4550年历史的前50年发生了变化，因此只反映了最初确定核和地幔之间成分差异的最古老的过程。W同位素系统对地幔中添加的岩芯物质也高度敏感，但初步研究表明，夏威夷样品中没有显示出最极端的Os同位素组成的岩芯W同位素特征。这里的工作旨在解决这一问题。需要为升高的186Os/188Os找到一个合理的替代方案，并需要充分解决对W同位素示踪剂可靠性提出的反对意见。此外，这一方法还需要进一步使用，而不是目前从一个海洋岛屿采集的三个样本的数据库。该项目将研究地球物理或地球化学证据表明岩浆活动的最终深源的大范围地点。此外，还将使用一种新的示踪剂，即Mo。利用对钼及其同位素浓度的分析，可以清楚地区分几种相互竞争的假设，以解释对比的W和Os同位素数据。这项研究的结果将提供一个明确的答案，即我们是否可以在地球表面对地球最深的储集层进行化学采样。如上所述，这不是无稽之谈，但将有助于我们了解构造板块中央的火山活动是如何发生的，以及在地球上最戏剧性的边界发生了什么过程。

Some 3000km beneath the surface of the Earth the temperature jumps 2000K and the surroundings change from solid silicate minerals to liquid iron alloy. This is the core-mantle boundary. It is our planet's most profound discontinuity, with huge physical and chemical changes occuring over a very small distance (<200m). The temperature contrast across the boundary has long been held responsible for heating the mantle above the core such that it becomes less dense and upwells as plumes to create oceanic islands such as Hawaii. However, this long held 'plume paradigm' has been increasingly questioned and some hard evidence for the an ultimately deep origin of the material that forms these oceanic islands is needed. The traditional tool for imagining the Earth's interior, seismology, has difficulty in clearly resolving narrow plumes beneath ocean islands. Geochemistry could provide the crucial evidence if a chemical inprint of residence next to the compositonally highly distinctive core could be found in oceanic island lavas. Recent work has suggested that there are such signatures. Initial work focussed on elevated values of an isotope ratio of the element Os, 186Os/188Os. Elevated 186Os/188Os ratios were found in lavas from Hawaii and can be attractively explained by a contribution from the core in their source. However there are other plausible explanations of this observation. A much less equivocal fingerprint comes from another isotope ratio, this time of the element W, 182W/184W. This ratio changed only in the first 50My of Earth's ~4550My history and so only reflects the most ancient processes that originally established the compositional differences between core and mantle. The W isotope system is also highly sensitive to the addition of core material to the mantle but an initial study indicated no core W isotope signature in the samples from Hawaii which showed the most extreme Os isotope compositions. The work here proposes to resolve this empasse. A plausible alternative needs to be found for the elevated 186Os/188Os and objections raised to the reliability of the W isotope tracer need to be fully addressed. Moreover, the approach needs to be used further than the current database of three samples from a single ocean island. This project will study a large range of locations where geophysical or geochemical evidence has suggested an ultimately deep source for magmatism. In addition a new tracer will be used, namely Mo. Using analyses of the concentration of Mo and its isotopes it is possible to clearly distinguish between several competing hypotheses to account for the contrasting W and Os isotopic data. The outcome of the study will provide a definitive answer as to whether we can chemically sample the Earth's deepst reservoir at the planet's surface. As indicated above, this is not idle speculation but will help us understand how volcanism in the middle of tectonic plates occurs and indeed what processes occur at the most dramatic boundary on Earth.